The need to quantitatively determine the amount of nitrogen and other elements contained in various organic compounds has been widespread for more than one hundred years. The need for determining the amount of nitrogen present led Johann Kjeldahl of Denmark to develop a method involving heating the substance in concentrated sulfuric acid and then adding powdered potassium permanganate. After addition of the permanganate the solution was diluted, transferred to a distilling flask, made alkaline, zinc added, and then distilled into standard acid. Potassium iodide and iodate were added to the distillate, and the liberated iodine was titrated with standard thiosulfate. See The Kjeldahl Method for Organic Nitrogen, R. B. Bradstreet (1965).
Others later learned that the speed of reaction in concentrated sulfuric acid was accelerated by the use of catalysts, such as the oxides of iron, mercury, manganese, bismuth, zinc, lead, and copper. Selenium was also touted as a good catalyst in the method, as was selenium dioxide-mercuric sulfate (1:1) and selenium dioxide-copper sulfate (3:1).
Because Kjeldahl's method required a considerable amount of time to digest the substance being tested, there was a desire to shorten the digestion time. This was eventually made possible when it was discovered that the addition of potassium sulfate raised the boiling point of the digestion mixture. Later it was found that the addition of various other sulfates and phosphates were also effective in raising the boiling point.
Hydrogen peroxide is reported to have been used in the Kjeldahl method as an oxidizing agent but with only limited success in aiding the digestion process. One researcher recommended the use of 30% hydrogen peroxide by mixing it with the organic matter to be treated, after which concentrated sulfuric acid is added slowly with shaking. Then potassium sulfate is added, and the mixture is then boiled. Modifications of the method by others involved adding various co-catalysts.
In another variation it was suggested to successively add 1-5 drops of 30% hydrogen peroxide to the carbonized digest, heat the digest until fumes appear, and reheat the digest after each addition. In yet another variation it was suggested to add peroxide after first heating the digest for five minutes over low flame. Then additional acid, copper sulfate catalyst and potassium sulfate are added and the resulting mixture heated until fumes are given off. Peroxide is then added until the solution remains blue and the digest is then heated for an hour with a high flame. Another suggested adding bromine after the organic matter has charred, reheating the digest, and again adding bromine with several drops of hydrogen peroxide; later peroxide is added alone.
It has also been suggested to heat the organic matter with oleum, then cool the digest after charring, followed by addition of peroxide to the cold digest. The digest is then heated for five minutes and the procedure repeated.
In yet another procedure concentrated sulfuric acid is added to the sample of organic matter and then allowed to stand for 15 minutes at room temperature before heating for several minutes. After cooling the digest, there are added 10-20 ml. of peroxide in small portions so as to avoid a large evolution of gas. After heating to expel the gas, the digest is boiled for five minutes and then the procedure is repeated until a clear solution results.
The various complexities and problems associated with the use of oxidizing agents in the Kjeldahl process have led researchers to conclude that it is better to depend upon the higher temperatures obtained by salt addition and an accelerated reaction rate through the use of catalysts than upon the use of an oxidizing agent to promote oxidation of organic matter. See p. 42 of The Kjeldahl Method for Organic Nitrogen, supra. The use of hydrogen peroxide has not been very satisfactory because of the time consuming, tedious nature of its application and only partial success in improving the Kjeldahl method. None of the previous methods using hydrogen peroxide have been proven to be faster or more accurate than other conventional Kjeldahl methods.
In the prior methods which utilized peroxide or peroxysulfuric acid, the concentration of the peroxide or peroxysulfuric acid was not maintained for sufficient time at a sufficiently high temperature to obtain any significant oxidation. The addition of hydrogen peroxide solution alone to hot sulfuric acid results in a violent decomposition at the surface of the acid layer with little benefit toward oxidation of the organic material dissolved in the sulfuric acid.
In U.S. Pat. No. 4,229,180 (Christofferson) there is described a process for determining nitrogen in a sample, according to the Kjeldahl principle, in which an antimonate compound is used as a catalyst. Mention is made at Column 2 that the antimonate may be added as a powder, granulate, tablet, or as a solution in water or in a component which is to be added in the destruction anyway, such as hydrogen peroxide or sulfuric acid or mixtures thereof. Such patent does not describe pre-digesting the sample in sulfuric acid. Apparently the hydrogen peroxide is added, with concentrated sulfuric acid, to the sample along with the catalyst and salt (for increasing the boiling point) before the sample is heated. Then the destruction mixture is heated to 400.degree. C. to 410.degree. C., during which water and peroxide are evaporated or consumed.
Unfortunately, when the digest is heated at 410.degree. C. or more there is a loss of nitrogen from the digest. As a result, the quantitative determination of nitrogen in the final digest will be erroneous. Also, when various salts are added to the digest in order to increase the boiling point of the sulfuric acid, such salts remain in the digest and prevent the digest from being used for quantitative determinations of elements (such as potassium) which are present in the added salts. Of course, the presence of catalyst has the same effect. Christofferson's method, using both catalysts and salts, requires temperatures and digestion times well outside that required in the process of the present invention (which does not use either catalysts or salts).
Yet another procedure is described in Analytical Chemistry, 20, pp. 481-488 (1948) in which the material sample is digested in sulfuric acid for five minutes, then cooled, after which two drops of hydrogen peroxide are added. Then the digest is heated again for two minutes and then cooled, after which another two drops of hydrogen peroxide are added. This process may be repeated several times. The process is tedious and time-consuming. The digestion time required for complete nitrogen recovery using such a process is several times longer than that obtainable by the process of the present invention.
Possibly the most advanced study of the use of peroxide is described in Australian J. Chem., 7, pp. 55-70 (1954) which reported only partial success, with difficulty, in using peroxide. The article reports that a fifty-minute digestion (with sulfuric acid and peroxide) of the amino acid tryptophan resulted in only 98.3% recovery of nitrogen, and that digestion included ten separate additions of peroxide.
As stated in Analytical Chemistry, Vol. 23, No. 11, p. 1634 (1951), the temperature attained in digestion of the sample in the Kjeldahl procedure has been of prime importance. Too low a temperature either requires too long a digestion time or fails to give good results. Too high a temperature may result in loss of nitrogen from the digest. For samples containing nitrogen in pyridine ring structures, digestion at temperatures below 370.degree. C. did not give quantitative recovery of pyridine with one hour digestion. At temperatures above 410.degree. C. it was reported that nitrogen may be lost.
In Industrial and Engineering Chemistry, Vol. 17, pp. 437-8 (1945) it was reported that a digestion time of 2-4 hours was required for compounds such as pyridine, nicotine and nicotinic acid, using various types of catalysts.
In the Official Methods of Analysis of the Association of Official Analytical Chemists, the most commonly-used official method, it is recommended that any sample containing organic material be digested for at least two hours.
Such prior procedures are very undesirable for many situations, particularly where very accurate results are required or where time is of the essence in obtaining the results. Also, prior procedures are cumbersome or tedious. As a result, the person conducting the testing must follow the prescribed procedure carefully to avoid mistakes.
There has not heretofore been provided a technique for the rapid and accurate digestion of samples which produces a digest suitable for determination of nitrogen and for other elements also.